The field of the invention is systems and methods for magnetic resonance imaging (“MRI”). More particularly, the invention relates to systems and methods for magnetic resonance image reconstruction.
Magnetic resonance imaging (“MRI”) typically requires long data acquisition times. One of the drawbacks of these long data acquisition periods is the possibility that motion artifacts can be introduced by the patient moving during the imaging scan. Long acquisition times also make it difficult to image moving organs. Breathing motions can be mitigated by acquiring data while the patients hold their breath; however, for long scan times this approach can cause discomfort for patients.
Some of the widely used MRI protocols require rapid data acquisitions to enable time-lapse imaging of biologic functions, such as real-time cardiovascular imaging or functional MRI (“fMRI”). Several rapid imaging techniques have been developed to address these needs; however, each of these imaging techniques has its own advantages and disadvantages. As a result of the still existing drawbacks of existing rapid imaging techniques, there remains a need for very high frame rates for cine MRI.
Currently the most widely used technique for rapid MRI is echo-planar imaging (“EPI”), which acquires data with periodic cycling of magnetic gradient fields after a single radio frequency (“RF”) excitation. However, EPI has several drawbacks, such as geometric distortions due to low bandwidth in the phase-encoding direction, image blurring due to T*2 relaxation, and signal dropouts due to magnetic susceptibility variations. Moreover, EPI increases the risk of peripheral nerve stimulation in the patient because of the sequence of rapidly switching frequency-encoding gradients. When peripheral nerve stimulation occurs, it is generally experienced as a mild, albeit uncomfortable, vibratory sensation of the skin; however, in some instances it can be experienced as a painful response. Therefore, it is desirable to avoid peripheral nerve stimulation in the interest of patient comfort. Also, EPI produces very loud acoustic noise for extended periods, which is another source of patient discomfort.
More recently, parallel imaging techniques were introduced as an alternative for accelerating image acquisition by omitting several phase-encoding steps and using complementary information from an array of RF receiver coils. The distinct RF coil sensitivity profiles, B1−, can be used to synthesize missing information in the image domain, as in sensitivity encoding (“SENSE”) techniques, or in k-space, as in simultaneous acquisition of spatial harmonics (“SMASH”) and generalized auto calibrating partially parallel acquisitions (“GRAPPA”) techniques. Parallel imaging techniques are capable of reducing the geometric distortions and image blurring present in EPI. However, the acceleration achieved by parallel imaging comes at the expense of reduced and nonuniform distribution of signal-to-noise ratio (“SNR”). The decrease in SNR is characterized by the so-called geometry factor (“g-factor”), which may lead to regions of very poor SNR. The g-factor is dependent on RF receive coil geometry and the amount of acceleration. Therefore, acceleration rates are usually kept at R=3 or lower because the SNR penalty increases progressively with increasing acceleration rate.
Several new parallel imaging methods have been introduced that utilize non-Cartesian gradient encoding schemes. Gradient fields with spherical or cylindrical geometry are typically used in these methods; these gradient fields usually have multiple field maxima and minima. This introduces spatial encoding ambiguity, which can be resolved using coil sensitivity profiles. One example of such a technique is known as parallel imaging technique using localized gradients (“PatLoc”). The major advantage of this technique is the reduced gradient field magnitudes inside the tissues, which allows faster switching of gradient fields. The PatLoc method has drawbacks, however. These drawbacks include a spatially varying point spread function (“PSF”) and technical challenges in implementing the non-Cartesian gradient coils. In a related method referred to as “O-space imaging,” second-order nonlinear gradient fields are used as encoding gradients, and the projections of the object onto sets of concentric rings are obtained for the reconstruction. One of the main disadvantages of O-space imaging and PatLoc imaging is the requirement of additional nonlinear gradient fields, which are not available in conventional MRI scanners. Additionally, because of the spatially-varying PSF, the spatial resolution varies by location inside the MRI system.
It would therefore be desirable to provide a method for accelerated magnetic resonance imaging (“MRI”) that utilizes conventional linear gradients and high-quality, uniform spatial resolution with minimal image artifacts.